The present invention relates to a liquid crystal display device and a liquid crystal display apparatus using the liquid crystal display device. Particularly, this invention relates to a reflective liquid crystal display device and a liquid crystal display apparatus using the reflective liquid crystal display device.
Projection liquid crystal display apparatuses, such as, a projector and a projection TV, are popular for displaying images on a large screen at high definition.
Brightness of images is one of the major factors in display performance in the projection liquid crystal display apparatuses. Higher aperture and higher reflectivity of pixel electrodes are a possible solution for brighter images in the liquid crystal display device of the projection liquid crystal display apparatuses.
The liquid crystal display device of the projection liquid crystal display apparatus is generally classified into a transparent type and a reflective type. The transparent type allows read light incident at one side of the device to pass through and emits the light at the other side, as display light. The reflective type reflects read light incident at one side of the device and emits the light at the same side, as display light. The reflective type is advantageous over the transparent type in brightness of images for its higher aperture than the latter type.
Brighter images require the reflectivity of 90% or higher for pixel electrodes. Such a high reflectivity is achievable according to, for example, Japanese Un-examined Patent Publication No. 2004-12670 (referred to as reference 1, hereinafter). The reference 1 discloses a pixel electrode of a multi-layered structure having a layer including aluminum and another layer including silver formed on the former layer, to achieve higher reflectivity.
Disclosed in Japanese Un-examined Patent Publication No. 11-344726, corresponding to U.S. Pat. No. 6,493,052, (referred to as reference 2, hereinafter) is another type of a pixel electrode with a multi-layered structure to achieve higher reflectivity. The multi-layered structure has a low refractive film formed on the pixel electrode and a high refractive film formed on the former film. The low refractive film is made of acrylic resin, polyimide, magnesium fluoride or silicon dioxide. The high refractive film is made of titanium dioxide, zirconia, ITO (indium tin oxide), silicon nitride or cerium dioxide.
The silver-made pixel electrode disclosed in the reference 1, however, raises a material cost for its expensive material of silver which is also hard to process (etching, etc.), thus improvements being required.
Experiments by the inventors of the present invention revealed the difficulty in production management in the reference 2 due to dependency of an offset voltage to the material of a high refractive film formed in the uppermost layer of the multi-layered structure, thus improvements being required.
Further experiments by the inventors of the present invention revealed disadvantages of the multi-layered structure disclosed in the reference 2: difficulty in achieving higher reflectivity in specific frequency ranges, particularly, in a blue wavelength range (for example, ranging from 430 nm to 460 nm) for the high refractive film discussed above, depending on the film material, such as, silicon nitride (SiN); and variation in offset voltage to extended exposure to light, particularly, of high energy emission in a blue wavelength range, causing burning to the liquid crystal display device.